It is known that in all the most modern high speed looms, the weft yarn has to be fed to the loom at the lowest and most regular tension, this task being performed by devices, also called weft feeders, which unwind the weft yarn from the feed bobbins and wind a small reserve thereof on a drum, from which said yarn is picked by the weft insertion members of the loom.
It is also known that in some types of more modern looms, as air or water looms, the weft feeding members are provided with means for detecting the yarn amount being drawn, so as to feed the loom with the right weft length to be inserted at each beating up.
Said detecting means usually comprise a group of photoelectric cells, through which it is possible to read and count the number of turns (or turn fractions) outgoing from the winding unit. The proper working of the device essentially depends on the exact reading and counting of the turns by the photoelectric cells. This is not always easy to obtain as, in practice, it happens very often that one has to operate in the presence of dust flakes which, passing through the field of action of the photoelectric cells, cause undesired signals and hence improper working of the device. If a dust flake interferes with the detecting unit and crosses the light beam of a photoelectric cell, this latter can count it as an outgoing turn.